1. Field of the Invention
The present invention relates to a tunable light source and the like used in a WDM (Wavelength Division Multiplexing) transmission system and the like, for example.
2. Description of the Related Art
Going into an era of broadband, there have been increasing adoptions of WDM transmission systems that are capable of achieving communication with a plurality of light wavelengths by a single system for enabling effective utilization of optical fibers. Recently, there has been widespread use of a DWDM device (Dense Wavelength Division Multiplexing device) which multiplexes some tens of light wavelengths and enables transmission at still higher speed. In accordance with this, each WDM transmission system requires light sources corresponding to each light wavelength, and the required number has been increased dramatically in accordance with high multiplexing. Furthermore, ROADM (Reconfigurable optical add/drop multiplexers) which adds/drops an arbitrary wavelength at each node have lately been investigated for commercial use. With adoption of the ROADM system, in addition to expanding the transmission capacity by multiplexing the wavelengths, it is possible to switch optical paths by changing the wavelengths. Therefore, flexibility of the optical network can be dramatically improved.
As the light source of the WDM transmission system, DFB-LD (Distributed feedback laser diode) which oscillates at uniaxial mode has been used widely so far due to its user-friendliness and high reliability. The DFB-LD has diffraction gratings of about 30 nm depth formed over the entire region of a resonator, so that stable uniaxial mode oscillation can be achieved with a wavelength corresponding to the product of the diffraction grating period and twice the equivalent refractive index. However, the resonant wavelengths over a wide range cannot be tuned in the DFB-LD, so that the WDM system is constituted using products which differ only in terms of the wavelengths for each ITU grid. Since it is necessary to use different products for each wavelength, management cost is increased and surplus stock is required in case of breakdown. Furthermore, when a normal DFB-LD is used in the ROADM system which switches the optical paths by the wavelengths, the tunable width of the wavelength range due to temperature variation is limited to about 3 nm. Therefore, it becomes difficult to achieve the structure of the optical network utilizing the characteristic of the ROADM that actively uses the wavelength resources.
In order to overcome the issues of current DFB-LD and achieve uniaxial mode oscillation in a wide range of wavelengths, there have been actively carried out researches of tunable lasers for tunable light sources. Hereinafter, some of the conventional tunable lasers among those described in detail in the following Non-patent Literature 1 will be presented as an example for describing a conventional tunable laser.
The tunable lasers are classified into two types, i.e. a type where a tuning mechanism is provided within a laser element, and a type where a tuning mechanism is provided outside the laser element.
As the former type, DBR-LD (Distributed Bragg Reflector Laser Diode) has been proposed. The DBR-LD has a structure in which an active region for generating gain and a DBR region for generating reflection by the diffraction grating are formed within a same laser element. The tunable range of the wavelengths of the DBR-LD is about 10 nm at the maximum. Further, there has been proposed a DBR-LD using non-uniform diffraction grating. This DBR-LD has a structure in which an active region for generating the gain and DBR regions sandwiching the active region from the front and rear are formed within a same laser element. In the DBR regions in the front and rear, a large number of reflection peaks are generated by the non-uniform diffraction grating, and the intervals between the reflection peaks are slightly shifted in the front and rear. Due to this structure, so-called “Vernier effect” can be achieved so that it is possible to perform an extremely wide range of tuning. The DBR-LD using the non-uniform diffraction grating achieves tuning action over 100 nm and quasi-continuous tuning action of 40 nm.
The tunable laser of the latter type has a structure that returns light of a specific wavelength to the laser element by rotating a diffraction grating provided outside the laser element.
[Non-patent Literature 1] “Optical Integrated Device” by Kohroh Kobayashi, 2nd Impression of 1st Edition, KYORITSU SHUPPAN CO., LTD, December 2000, pp. 104-122
[Non-patent Literature 2] “Optical Filter Design and Analysis” by C. K. Madsen, J. H. Zhao
However, even though there have been a great number of structures proposed as the conventional tunable lasers, there faces an issue that an unexpected switching of wavelengths called mode jump is likely to be generated. Further, in addition to the fact that it requires complicated wavelength control, there are shortcomings such as low oscillation resistability, high price due to an increase in the number of elements, etc. Therefore, there still remains in the condition that it is difficult to be put in a practical use.
In the DBR-LD, carrier injection is performed to the DBR region for changing the refractive index in the DBR region so as to change the wavelength range. Thus, if crystal defects increase due to the injection of the electric current, the proportion of changes in the refractive index for the current injection fluctuates strikingly. Therefore, it is difficult to maintain the laser oscillation with a constant wavelength. Furthermore, with the current process technique of a compound semiconductor, inch-up of three inches or more is impossible, and there requires a tremendous amount of development cost for achieving it. Thus, the manufacturing cost largely increases with the laser element that is in a structure required to be large-scaled.
In the laser element where the tuning mechanism is provided outside, mode jump is easily generated by the oscillation. Thus, it requires a large-scaled oscillation-resistant mechanism, which results in large-scaled module size and increased price.